Carbon dioxide production from combustion exhaust gases with nitrogen and argon by-product recovery

ABSTRACT

The present invention is directed to a method for producing carbon dioxide and nitrogen from combustion exhaust gas containing less than about 10% oxygen by weight which comprises the steps of (a) treating the exhaust gas to remove particulate matter, (b) compressing the exhaust gas to a pressure in the range from about 25 psia to about 200 psia, (c) purifying the exhaust gas to remove trace contaminants, (d) separating the exhaust gas to produce a carbon dioxide rich fraction and a nitrogen rich fraction, (e) liquifying the carbon dioxide rich fraction and distilling off volatile contaminants to produce pure carbon dioxide, (f) purifying the nitrogen rich fraction to remove contaminants, and (g) cryogenically fractionally distilling the nitrogen rich fraction to produce pure nitrogen. In another embodiment, the invention is directed to a method for producing carbon dioxide, nitrogen, and argon from a combustion exhaust gas. The combustion exhaust gas in the present invention may be obtained from an ammonia plant reformer furnace and the nitrogen produced may be employed as a synthesis gas in the ammonia reactor.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention is directed to a method for producing carbondioxide, nitrogen, and optionally argon, from a combustion exhaust gas.More particularly, the present invention is directed to a method forseparating carbon dioxide from an oxygen depleted combustion exhaust gasto produce a feed gas enriched in nitrogen and argon.

2. Description of the Prior Art

The commercial preparation of carbon dioxide and nitrogen is well knownin the art. Carbon dioxide is normally produced as a by-product fromchemical processes for producing ammonia, hydrogen, ethanol, ethyleneoxide, and gasoline, as well as in fermentation reactions and carbonatedecompositions. Nitrogen is generally produced by separation from air.

The preparation of carbon dioxide generally involves the steps of crudegas generation, purification and separation, compression andliquefaction, drying, and rectification distillation.

Generation of crude carbon dioxide involves the combustion of liquidfuels such as fuel oil, or solid fuels such as anthracites, coke,charcoal, and the like, with excess air to promote complete oxidation ofthe fuel and to provide a carbon dioxide rich combustion exhaust gas.

Purification of the combustion exhaust gas generally involves severalseparate treatments to provide a gas having high purity. Thesepurification treatments include washing, absorption, adsorption,desorption, and the removal of reducing substances. Washing generallyinvolves a water absorption shower (water wash) to remove solids (soot,carried off ashes, etc.) and at the same time to cool the combustiongases. Various scrubbing solutions are generally employed to removecontaminants and to reduce the components in the combustion gas mixtureto carbon dioxide, nitrogen, and oxygen. The combustion exhaust gas mayalso be passed through a tower containing a recirculating oxidizingsolution such as potassium permanganate to remove traces of organicimpurities carried with the gas.

The washed and scrubbed combustion gas is then separated to obtain acarbon dioxide rich fraction. In one separation method, the combustiongas mixture is circulated through a counter-current shower of anabsorbing solution such as potassium carbonate, monoethanol-amine, andthe like. Carbon dioxide can be desorbed by heating the carbon dioxidesaturated solution to a temperature above 100° C. In another separationmethod, the combustion mixture is separated by selectively adsorbing thecarbon dioxide on a zeolite bed in a pressure swing adsorption system.

The purified and separated carbon dioxide is then compressed to apressure in the range from about 230 psia to about 400 psia, dried bycontacting the gas with a regenerable desiccant, and liquified bylowering the temperature of the gas. Finally, a rectificationdistillation step eliminates the small amount of nitrogen, oxygen, andargon to provide carbon dioxide having a purity of about 99.9% byvolume.

The most common methods for separating nitrogen from air are cryogenicfractional distillation, inert gas generation (combustion of natural gasor propane in air), and pressure swing adsorption.

In cryogenic fractional distillation, air is compressed to about 100 psiand cooled in a reversing heat exchanger against outgoing nitrogenproduct gas and waste gas. Water, carbon dioxide, and hydrocarbons inthe air are removed by condensation in the reversing heat exchanger.Alternatively, water, carbon dioxide, and hydrocarbons can be removed bypassing air through a zeolite bed. The zeolite bed can be regenerated bypassing heated nitrogen waste gas through the bed. The air is fedthrough a cold end gel trap where remaining small quantities ofhydrocarbons and carbon dioxide are removed. The clean air is cooledfurther in a sub-cooler and is fed into a distillation column where theair is liquefied and separated into a high purity nitrogen product gasfraction and a waste gas fraction containing about 38% oxygen by weight.Both gas fractions are warmed to ambient temperature by passing thefractions through the sub-cooler and reversing heat exchanger.

In an inert gas generator, natural gas or propane is burned with air andthe products of combustion are removed leaving purified nitrogen. Thecombustion of natural gas and air is controlled to provide a specificair to gas ratio in the burner to obtain essentially completecombustion. The combustion gas contains nitrogen, carbon dioxide, watervapor, and small amounts of carbon monoxide and hydrogen. Gases leavingthe combustion chamber are cooled in a surface condenser to removewater. The gases then flow to a refrigerant dryer where the dew point isreduced to 4° C. Pure nitrogen product gas is then obtained by passingthe gas through a molecular sieve bed in a pressure swing adsorptionapparatus to remove carbon dioxide and any remaining water vapor.

In a pressure swing adsorption system (PSA), air is passed at anelevated pressure through a bed of an adsorbent material whichselectively adsorbs oxygen. Nitrogen product gas is then withdrawn fromthe bed. The adsorption bed may be regenerated by reducing the pressureof the bed.

U.S. Pat. No. 3,493,339, issued to Weir et al., discloses a method forproducing carbon dioxide and separating argon which comprises combustinga carbonaceous material in a mixture of argon and oxygen and separatingthe combustion products to obtain carbon dioxide and argon.

U.S. Pat. No. 4,414,191, issued to Fuderer, discloses a pressure swingadsorption method for purifying hydrogen for ammonia synthesis. Nitrogenat elevated pressure is used as the purge gas in the pressure swingadsorption separation and the nitrogen in the purified gas is employedin the ammonia synthesis stream.

U.S. Pat. No. 4,797,141, issued to Mercader et al., discloses a methodfor obtaining carbon dioxide and nitrogen from the oxygen rich exhaustgas of an internal combustion engine or turbine. The method comprisesthe steps of cooling the exhaust gas, separating carbon dioxide from thecooled gas by absorbing the carbon dioxide in an alkaline solution,recovering the carbon dioxide by liberating the gas from the carbonatedsolution, compressing and liquifying the carbon dioxide, recovering thenitrogen by purifying the gas to remove contaminants, and compressingand liquifying the nitrogen.

While the above methods provide improvements in the production of carbondioxide, none of these methods are entirely satisfactory. Conventionalsources for producing carbon dioxide are carbon dioxide rich gases suchas waste gases from ammonia, hydrogen, ethanol, and ethylene oxideplants. These carbon dioxide sources are not always available or are notalways reliable especially at locations of high carbon dioxide demand.Other common problems with the production of carbon dioxide are lowproduct yield and energy inefficient separation methods. Conventionalgas generation methods do not teach the preparation of food grade carbondioxide as well as pure nitrogen and argon from combustion exhaustgases. Hence there is a need for an improved method for producing carbondioxide. The present invention provides such an improved method and alsoprovides an improved method for producing nitrogen and argon asby-products.

SUMMARY OF THE INVENTION

The present invention is directed to a method for producing carbondioxide and nitrogen from combustion exhaust gas containing less thanabout 10% oxygen by weight which comprises the steps of (a) treating theexhaust gas to remove particulate matter, (b) compressing the exhaustgas to a pressure in the range from about 25 psia to about 200 psia, (c)purifying the exhaust gas to remove trace contaminants, (d) separatingthe exhaust gas to produce a carbon dioxide rich fraction and a nitrogenrich fraction, (e) liquifying the carbon dioxide rich fraction anddistilling off volatile contaminants to produce pure carbon dioxide, (f)purifying the nitrogen rich fraction to remove contaminants, and (g)cryogenically fractionally distilling the nitrogen rich fraction toproduce pure nitrogen. In another embodiment, the invention is directedto a method for producing carbon dioxide, nitrogen, and argon from acombustion exhaust gas. The combustion exhaust gas in the presentinvention may be obtained from an ammonia plant reforming furnace andthe nitrogen produced may be employed as a synthesis gas in the ammoniareactor.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a schematic process flow diagram illustrating a method for theco-production of carbon dioxide and nitrogen according to the method ofthe present invention.

FIG. 2 is a schematic process flow diagram illustrating a method for theproduction of carbon dioxide, nitrogen, and argon according to themethod of the present invention.

FIG. 3 is a schematic drawing of an apparatus suitable for integratingthe recovery of the production of carbon dioxide, nitrogen, and argonfrom the combustion exhaust gas of an ammonia reforming furnace into anammonia synthesis plant.

DETAILED DESCRIPTION OF THE INVENTION

Applicants have found that the production of carbon dioxide from acombustion exhaust gas (stack gas) containing less than about 10% oxygenby weight provides a method which efficiently and economically yieldsenriched carbon dioxide in high purity. After removal of tracecontaminants from the oxygen depleted gas, liquid carbon dioxide isproduced by bulk separation, liquefaction, and distillation of volatileimpurities. Nitrogen, and optionally argon, are then recovered from thecarbon dioxide depleted gas as b-products by cryogenic fractionaldistillation. The reduced oxygen concentration in the combustion exhaustgas provides process flexibility and capital cost reduction.

After carbon dioxide is separated from the stack gas, the concentrationof nitrogen and argon in the stack gas is significantly higher than theconcentration in air, the conventional source of these gases. Thishigher nitrogen and argon concentration is the result of oxygen beingconsumed in the combustion process. Separation of nitrogen and argon asby-products from the carbon dioxide depleted gas results in asignificant reduction in energy (about 40%) compared to production fromair separation. The present method provides an opportunity for reducingthe cost for manufacturing liquid carbon dioxide and makes combustionexhaust gas a viable and attractive carbon dioxide source.

The gaseous nitrogen product obtained by the present method may be usedas a synthesis gas or as an inert gas at the chemical plant whichprovides the combustion exhaust gas, such as a hydrogen plant or arefinery. Alternatively, the nitrogen product may be liquefied fordistribution to other locations. The reduction in feedstream cost anddistribution cost also offset the cost of bulk separation required toconcentrate the relatively low carbon dioxide content of the combustionexhaust gas and the processing cost to remove trace contaminants such asnitrogen oxides (NO_(x)) and sulfur oxides (SO_(x)). Conversion ofcontaminants in the combustion exhaust gas to an easily disposable formand separation and recovery of the components also provides an efficientand attractive option to meet clean air regulations and environmentalcontrol.

In a preferred embodiment, the recovery of combustion exhaust gas froman ammonia plant reforming furnace is integrated with the synthesisprocess in the ammonia plant. A conventional method for producingammonia is based on the primary steam reforming of natural gas or otherhydrocarbon gas followed by secondary reforming with air to provide ahydrogen and nitrogen synthesis gas mixture. Contaminants such as carbonmonoxide are removed by shift conversion (conversion of carbon monoxidewith steam to form additional hydrogen and carbon dioxide) andcontaminants such as carbon dioxide are removed by absorption in aminesor other alkaline solvents. Carbon monoxide and carbon dioxide are alsoremoved by methane formation (conversion of trace carbon monoxide andcarbon dioxide to methane). The purified hydrogen and nitrogen synthesisgas mixture is then fed into the ammonia synthesis reactor.

A more recent method for producing ammonia is based on the production ofpure hydrogen synthesis gas by steam reforming and on the production ofpure nitrogen synthesis gas by separation of air. The production ofhydrogen gas consists of steam reforming, carbon monoxide shiftconversion and multiple bed pressure swing adsorption purification.

In a preferred embodiment, hydrogen gas is produced by steam reforming,shift conversion and pressure swing adsorption purification and is mixedwith nitrogen gas recovered from the carbon dioxide depleted combustionexhaust from the ammonia plant steam reformer furnace. The hydrogen andnitrogen synthesis gas mixture reacts in the ammonia plant synthesisreactor to form ammonia. Furthermore, the carbon dioxide separated formthe stack gas in the reforming step can be combined with the ammoniaproduct gas in a urea plant to yield urea. Accordingly, the presentmethod can provide pure nitrogen synthesis gas and carbon dioxidesynthesis gas to yield product ammonia and urea at lower energy coststhan conventional technologies.

Ammonia production processes and hydrogen production processes aredisclosed in more detail in "Ammonia and Synthesis Gas: Recent andEnergy Saving Processes", Edited by F. J. Brykowski, Chemical TechnologyReview No. 193, Energy Technology Review No. 68, Published by Noyes DataCorporation, Park Ridge, N.J., 1981, which disclosure is incorporatedherein by reference.

In accord with the present invention, the method for producing carbondioxide and nitrogen from combustion exhaust gas containing less thanabout 10% oxygen by weight comprises the steps of (a) treating theexhaust gas to remove particulate matter, (b) compressing the exhaustgas to a pressure in the range from about 25 psia to about 200 psia, (c)purifying the exhaust gas to remove trace contaminants, (d) separatingthe exhaust gas to produce a carbon dioxide rich fraction and a nitrogenrich fraction, (e) liquifying the carbon dioxide rich fraction anddistilling off volatile contaminants to produce pure carbon dioxide, (f)purifying the nitrogen rich fraction to remove contaminants, and (g)cryogenically fractionally distilling the nitrogen rich fraction toproduce pure nitrogen.

The combustion exhaust gas in the present invention is a combustion gascontaining less than about 10% oxygen, preferably from about 1.5% toabout 6% oxygen, and more preferably from about 1.5% to about 3% oxygen,by weight. The combustion preferably takes place in a fired heater(steam boiler) under approximately stoichiometric conditions, andmoderation of the combustion can be achieved by recycle of some of thereaction products. A 10% excess of air to fuel is normally used in afired heater to ensure complete combustion of the fuel and this air tofuel ratio results in approximately 2% oxygen concentration by weight inthe stack gas.

Fuel such as natural gas, methane, coke, coal, fuel oil, or similarcarbon-containing compounds may be combusted with air. The fuel supplymay also be waste or exhaust gases from other sources. For example, in acombined cycle power plant, a gas engine or turbine may be initiallyused and the exhaust gas from the engine is further combusted in a firedheater with supplementary fuel to generate steam. The combustion exhaustgas may be obtained from a number of sources such as a power plant,cement and lime plants, and chemical plants such as ammonia plants andhydrogen plants. Chemical plant waste gases from refinery fluidcatalytic cracking unit regeneration gases and combustion exhaust gasfrom incinerators may also be used.

In general, combustion gases from internal combustion engines orturbines are not suitable in the present invention because such exhaustgases contain high amounts of oxygen making the gas separationuneconomical. Typically a combustion engine uses 70% to 300% excess airto ensure complete combustion of the fuel and to prevent the engine orturbine from overheating during the combustion process. This level ofexcess air means that the oxygen concentration in the exhaust gas willbe very high, typically about 17%. Because there is no substantialreduction in the oxygen concentration in the exhaust gas of an enginecompared to the oxygen concentration in air (about 20%), there is noappreciable energy or capital cost savings advantage for producingnitrogen from the carbon dioxide depleted exhaust gas from an enginecompared to the conventional production of nitrogen from air.

The method for producing carbon dioxide, nitrogen, and argon from acombustion exhaust gas can be better understood by reference to theFIGURES in which like numerals refer to like parts of the inventionthroughout the FIGURES. Although the present invention is described andillustrated in connection with preferred embodiments, applicants intendthat modifications and variations may be used without departing from thespirit of the present invention.

Referring to FIG. 1, combustion exhaust gas (stack gas, combustion gas,exhaust gas, feed gas, waste gas) is fed through gas feed conduit 1 topre-purification unit 2 to remove particulate matter from the combustionexhaust gas. Pre-purification unit 2 may be a washing column whereincombustion gas is admitted from the bottom of the unit and a waterabsorption shower is administered to the gas from the top of the unit toremove solids (soot, carried off ashes, etc.). The washing column may atthe same time cool the gas and remove sulfur anhydrides derived fromsulfur contained in the fuel. Heat obtained from the combustion gas maybe used to preheat the fuel gas in the fired heater.

The pre-purified combustion exhaust gas is then fed through gas feedconduit 3 to compressor 4. Compressor 4 compresses the combustion gas tothe separation pressure. In general, the combustion exhaust gas iscompressed to a separation pressure in the range from about 25 psia toabout 200 psia, preferably from about 25 psia to about 120 psia, andmore preferably from about 40 psia to about 100 psia.

The compressed combustion exhaust gas is then fed through gas feedconduit 5 to purification unit 6 where trace contaminants such asnitrogen oxides, sulfur oxides, and water are removed. For example,nitrogen oxides (NO_(x), NO, NO₂) may be removed by treating the feedgas with ammonia and a selective catalyst (commercially available, forexample, from Norton Company, Ohio) to convert the nitrogen oxides tonitrogen and water. Sulfur oxides (SO_(x), SO₂, SO₃) may be removed bytreating the feed gas with conventional flue gas desulfurizationtechniques such as alkali scrubbing. Other methods to remove nitrogenoxides and sulfur oxides include moving bed adsorption on activatedcarbon (Bergbau-Forschung process) and cyanuric acid treatment (RAPRENOxprocess developed by Sandia National Laboratories), respectively.Potassium permanganate scrubbing may also be included in thepurification to reduce trace contaminants such as NO_(x) to the desiredlevel. The presence of nitrogen oxides and sulfur oxides in thecombustion exhaust gas should be reduced to less than about 1 ppm tomeet food grade specifications for liquid carbon dioxide products.Levels of carbon monoxide in the exhaust gas at concentrations higherthan ambient can be removed by catalytic oxidative conversion to carbondioxide. Water vapor can be removed, for example, by passing the feedgas through a tower containing a regenerable desiccant such as silicagel, alumina, or zeolite. Silica gel may be periodically regenerated bypassing dry nitrogen heated to a temperature above 100° C. through thetower.

The purified combustion exhaust gas is then passed through gas feedconduit 7 to separation unit 8 to separate the gas to produce a carbondioxide rich fraction and a nitrogen rich fraction. The separation ofthe feed gas can be carried out by any conventional method.

In one embodiment, the combustion exhaust gas may be circulated throughcarbon dioxide absorption columns (alkaline solutions such asmonoethanolamine, potash, etc.) wherein carbon dioxide is absorbed toform a carbonated solution and nitrogen and the remaining gases passthough the column. The carbonate solution can be regenerated by passingsteam or fluid at a temperature of about 125° C. through the carbonatedsolution. In a preferred embodiment, the combustion exhaust gas isseparated in a pressure swing apparatus into a carbon dioxide richstream and a nitrogen rich stream.

The carbon dioxide rich fraction from separation unit 8 is then fedthrough gas feed conduit 9 to liquefaction unit 10 wherein the carbondioxide is liquified and the volatile contaminants are removed bydistillation to produce pure carbon dioxide. Liquid carbon dioxide isproduced by conventional processing steps that include compressing thegas to a pressure between about 230 psia and about 400 psia and coolingthe gas to a temperature between about -8° F. and about -50° F. The morevolatile impurities are removed from the liquid carbon dioxide bydistillation. Pure carbon dioxide is then vented from liquefaction unit10 through feed conduit 11 to carbon dioxide product reservoir 12.

The nitrogen rich fraction from separation unit 8 is then fed throughgas feed conduit 13 to nitrogen purification unit 14 wherein thenitrogen fraction is purified to remove trace contaminants. The nitrogenrich fraction from the bulk carbon dioxide separation in separation unit8 typically contains about 96% nitrogen, about 1.2% argon, and about2.8% oxygen, by weight. Preferably, the nitrogen rich fraction ispurified by passing the gas through a bed of zeolite molecular sieves toremove trace contaminants such as carbon dioxide.

Pure nitrogen gas is then generated by cryogenic fractionaldistillation. The nitrogen rich fraction from nitrogen purification unit14 is fed through gas feed conduit 15 to heat exchanger 16 where thefeed gas is cooled to close to its liquefaction point (with coolingenergy derived from the outgoing product gas stream). Cooled nitrogengas from heat exchanger 16 is fed through gas feed conduit 17 to feedexpander 18 where the nitrogen gas is further cooled and partiallyliquified (typically from about 10% to about 15% of the nitrogenfraction is liquified). Cooled nitrogen gas from feed expander 18 is fedthrough gas feed conduit 19 to nitrogen generator 20 where pure nitrogenis cryogenically fractionally distilled from oxygen and argon. Purenitrogen product gas passes from nitrogen generator 20 through gas feedconduit 21, gas mixing union 22, and gas feed conduit 23 to heatexchanger 16 where the product gas is brought to ambient temperature.Cooling energy from the pure nitrogen product gas is passed to heatexchanger 16 for cooling feed gas from nitrogen purification unit 14.Warmed product gas is then passed from heat exchanger 16 through gasfeed conduit 24, gas splitting union 25, and gas feed conduit 26 tonitrogen product reservoir 27. Gas feed conduits 21 and 23 are connectedby gas mixing union 22. Gas mixing union 22 is also connected to flashpot 28 via gas feed conduit 29. Gas feed conduits 24 and 26 areconnected by gas splitting union 25. Gas splitting union 25 is alsoconnected to nitrogen cycle compressor 30 via gas feed conduit 31.

A portion of the nitrogen product gas passes from gas feed conduit 24,gas splitting union 25, and gas feed conduit 31 to nitrogen cyclecompressor 30 to supply the refrigeration loop. Nitrogen cyclecompressor 30 compresses the nitrogen product gas into nitrogenrefrigeration fluid. The nitrogen refrigeration fluid is cooled andpartially liquified by passage to heat exchanger 16 via gas feed conduit32. The cooled nitrogen refrigeration fluid then passes to reboiler 33via gas feed conduit 34. Partially liquified nitrogen refrigerationfluid in reboiler 33 accepts cooling energy from reboiler 33. Afterbeing substantially liquified, the nitrogen refrigeration fluid fromreboiler 33 passes to flash pot 28 via feed conduit 35. Flash pot 28expands the nitrogen refrigeration fluid to a lower pressure to subcoolthe refrigeration fluid. Flash pot 28 separates liquified nitrogenrefrigeration fluid and gaseous nitrogen refrigeration fluid. Liquifiednitrogen refrigeration fluid from flash pot 28 is returned as reflux tonitrogen generator 20 via feed conduit 36. Gaseous nitrogenrefrigeration fluid from flash pot 28 is passed through gas feed conduit29 and gas mixing union 22 to join pure nitrogen product in gas feedconduit 23. After passage through heat exchanger 16 and gas feed conduit24, the product gas is again split at gas splitting union 25 betweennitrogen product reservoir 27 and nitrogen cycle compressor 30 to passinto the refrigeration loop.

Oxygen rich product gas in nitrogen generator 20 is vented from thebottom of nitrogen generator 20 via gas feed conduit 37 to heatexchanger 16 to provide cooling energy to the heat exchanger. The warmedgas from heat exchanger 16 is passed to nitrogen purification unit 14(zeolite bed) via gas feed conduit 38 to be used as regeneration gas.Optionally, the regeneration gas may be further warmed by a heater priorto use in purification unit 14. After regeneration of nitrogenpurification unit 14, the oxygen rich waste gas is then vented fromnitrogen purification unit 14 via gas feed conduit 39.

In another embodiment, the invention is directed at a method forproducing carbon dioxide, and nitrogen and argon as by-products fromcombustion exhaust gas containing less than about 10% oxygen by weight.After carbon dioxide is separated from the stack gas, the concentrationof nitrogen and argon in the stack gas is considerably higher than theconcentration in air, the conventional source of these gases. Separationof nitrogen and argon as by-products from the carbon dioxide depletedgas results in a significant reduction in energy and cost formanufacturing liquid carbon dioxide. Furthermore, the productcombination consisting of carbon dioxide, nitrogen, and argon may bemore attractive for certain plant locations where production of oxygenis not in great demand.

Referring to FIG. 2, the combustion exhaust gas is fed topre-purification unit 2 to remove particulate matter from the combustionexhaust gas, as set out above for FIG. 1. The pre-purified combustionexhaust gas is then fed to compressor 4 which compresses the combustiongas to the separation pressure. The compressed combustion exhaust gas isthen passed to purification unit 6 to remove trace contaminants. Thepurified gas is fed to separation unit 8 to separate the gas to producea carbon dioxide rich fraction and a nitrogen rich fraction. The carbondioxide rich fraction is then fed to liquefaction unit 10 wherein thecarbon dioxide is liquified by conventional means and the volatilecontaminants are removed by distillation to produce pure carbon dioxide.Pure carbon dioxide is fed to carbon dioxide product reservoir 12. Thenitrogen rich fraction is then fed to nitrogen purification unit 14wherein the nitrogen rich fraction (carbon dioxide depleted) is purifiedto remove trace contaminants such as carbon dioxide in a zeolite basedadsorption purification system. The nitrogen rich fraction from nitrogenpurification unit 14 is fed to heat exchanger 16 where the feed gas iscooled to close to its liquefaction point. Cooled nitrogen gas from heatexchanger 16 is fed to feed expander 18 where the nitrogen gas ispartially liquified. Cooled nitrogen feed from feed expander 18 is fedto nitrogen generator 20 where pure nitrogen is fractionated from argon,as set out above for FIG 1.

Nitrogen waste gas can be vented from nitrogen generator 20 through gasfeed conduit 45 located near the top of nitrogen generator 20, gasmixing union 41, and gas feed conduit 42 to heat exchanger 16 and gasfeed conduit 43 for regeneration of nitrogen purification unit 14 andventing through gas feed conduit 44. An argon rich fraction in nitrogengenerator 20 is vented from the middle of nitrogen generator 20 via gasfeed conduit 46 to heat exchanger 16 to warm the gas. Warmed argon richfraction gas is then passed to argon generator 48 through gas feedconduit 47.

In one preferred embodiment, argon generator 48 is a pressure swingadsorption unit. Argon generator 48 separates the argon rich fractioninto crude argon product and an oxygen rich fraction. Crude argonproduct from argon generator 48 is passed to argon product reservoir 49via gas feed conduit 50. The oxygen rich fraction containing argon fromargon generator 48 is passed to compressor 51 via gas feed conduit 52 tobe compressed. Compressed oxygen rich fraction is then passed to heatexchanger 16 via gas feed conduit 53 to be cooled and then to nitrogengenerator 20 via gas feed conduit 54 to recycle residual argon.

In another preferred embodiment, argon generator 48 is a secondcryogenic distillation unit (not shown in FIG. 2). When argon generator48 is a cryogenic distillation unit, the argon rich fraction gas is notwarmed prior to passing the gas into argon generator 48 and the oxygenrich fraction withdrawn from argon generator 48 is not cooled prior topassing the fraction into nitrogen generator 20.

Pure nitrogen gas and crude argon (98+% argon and less than 2% oxygen,by weight) can be generated by employing two cryogenic distillationcolumns or one cryogenic distillation column and a pressure swingadsorption apparatus utilizing a carbon molecular sieve adsorbent. Thefirst cryogenic distillation column fractionates the feed gas into apure nitrogen product of desired purity and an oxygen (and argon) richfraction. When two cryogenic distillation columns are employed, theargon in the feed gas is separated with the oxygen rich fraction in thefirst cryogenic distillation column and is fractionated in the secondcryogenic distillation column as a crude argon product. When onecryogenic distillation column and a carbon molecular sieve (CMS)pressure swing adsorption apparatus are employed, an argon rich fractionis withdrawn from the cryogenic distillation column and separated in thecarbon molecular sieve pressure swing adsorption apparatus into a crudeargon product and an oxygen rich waste fraction. The oxygen rich wastefraction is recycled to the cryogenic distillation column. The refluxfor the cryogenic column(s) is provided by a recirculating nitrogenstream which acts as a heat pump to recover the cooling energy from thereboiler. Additional cooling energy is generated by expansion of thecooled feed gas or a portion of the compressed and cooled recirculatingnitrogen.

In a preferred embodiment, the invention is directed at a method forproducing carbon dioxide, nitrogen, and argon from a combustion exhaustgas containing less than about 10% oxygen by weight which comprises thesteps of:

(a) treating the exhaust gas to remove particulate matter;

(b) compressing the exhaust gas to a pressure in the range from about 25psia to about 200 psia;

(c) purifying the exhaust gas to remove trace contaminants;

(d) separating the exhaust gas to produce a carbon dioxide rich fractionand a nitrogen and argon rich fraction;

(e) liquifying the carbon dioxide rich fraction and distilling offvolatile contaminants to produce pure carbon dioxide;

(f) purifying the nitrogen and argon rich fraction to removecontaminants;

(g) cryogenically fractionally distilling the nitrogen and argon richfraction to produce pure nitrogen and an argon rich fraction; and

(h) purifying the argon rich fraction to produce pure argon.

In another embodiment, the invention is directed at an improved methodfor the production of ammonia. Nitrogen gas, recovered from the carbondioxide depleted combustion exhaust gas from the ammonia plant steamreformer furnace according to the method of the present invention, canbe utilized in the ammonia plant as a synthesis gas with hydrogensynthesis gas produced by steam reforming, shift conversion and pressureswing adsorption purification.

Steam reforming, in the hydrogen production process, consists oftreating a hydrocarbon feed gas with steam in a catalytic steam reactor(reformer) which consists of a number of tubes placed in a furnace at atemperature in the range from about 1400° F. to about 1700° C. Thereforming reactions which occur when methane is used as the hydrocarbonfeed gas are set out below.

    CH.sub.4 +H.sub.2 O=CO+3H.sub.2

    CH.sub.4 +2H.sub.2 O=CO.sub.2 +4H.sub.2

    CO+H.sub.2 O=CO.sub.2 +H.sub.2

The hydrogen rich gas mixture exiting the steam reformer consists of anequilibrium mixture of hydrogen, steam, carbon monoxide, carbon dioxide,and small amounts of unreacted methane. The reforming reactions areendothermic and require heat. Therefore, some hydrocarbon and processwaste gases are burned in air in the reformer furnace to provide theendothermic heat for the reforming reactions as well to preheat the feedand steam mixture.

Heat is extracted from the hot synthesis gases by cooling the gases withboiler feed water to a temperature of about 750° F. in a process boiler.The boiler feed water is converted to steam.

The cooled hydrogen rich gas is then treated in a shift converter to aidin the conversion of carbon monoxide into additional hydrogen and carbondioxide. The shift conversion reaction is favored at lower temperaturessuch as about 750° F. compared to the higher temperatures in the steamreformer.

The gases exiting the shift reactor are cooled in a process cooler toambient temperature. The heat extracted from the gases is used to heatmake-up water to produce boiler feed water for the process boiler.Condensate is also removed from the synthesis gas and is cycled into themake-up water to provide the feed to generate boiler feed water.

After being cooled, the shift reactor gases are then treated in ahydrogen pressure swing adsorption purification unit to produce purehydrogen gas for ammonia synthesis. The pressure swing adsorption systemusually contains between 4 and 12 adsorption vessels and operates on aprocess sequence consisting of the following steps: (i) adsorption toadsorb impurities on the bed and release pure hydrogen, (ii) severalstages of pressure equalization to conserve hydrogen in the void gas atthe end of the adsorption step, (iii) depressurization and purge with aportion of the hydrogen product gas to regenerate the bed and to removeimpurities, and (iv) repressurization of the adsorption bed usingpressure equalization gas and finally product hydrogen. The gas mixturereleased in step (iii) which is referred to as hydrogen pressure swingadsorption purge gas is cycled to the reformer furnace for burning torecover fuel value.

Referring to FIG. 3, hydrocarbon feed gas is fed through gas conduit 55and steam is fed through gas conduit 56 to catalytic steam reformer(reactor) 57 containing catalyst tubes 58. Hydrocarbon fuel is fedthrough gas conduit 59 and air is fed through gas conduit 60 to thefurnace in catalytic steam reformer 57. A hot hydrogen rich gas mixtureexits catalytic steam reformer 57 through gas conduit 61 and passes intoprocess boiler 62 where heat is extracted from the hot synthesis gasesin process boiler 62. Boiler feed water is introduced into processboiler 62 via conduit 63 and steam is removed from process boiler 62 viaconduit 64. The cooled hydrogen rich gas is then passed into shiftconverter 66 via gas conduit 65 where carbon monoxide is converted intohydrogen and carbon dioxide. The gases exiting shift converter 66 arepassed via gas conduit 67 into process cooler 68 where heat is extractedfrom the gases in process cooler 68 and condensate is removed. Make-upfeed water is introduced into process cooler 68 via conduit 69 andheated boiler feed water is remove from process cooler 68 via conduit70. Condensate removed from the synthesis gas is then cycled into themake-up water via conduit 71 to provide the feed water to generateboiler feed water.

The cooled shift reactor exit gases are withdrawn from process cooler 68and passed via gas conduit 72 to hydrogen pressure swing adsorptionpurification unit 73 to produce pure hydrogen gas. Pressure swingadsorption purification unit 73 may contain between 4 and 12 adsorptionvessels. Hydrogen pressure swing adsorption purge gas mixture is ventedfrom hydrogen pressure swing adsorption purification unit 73 via gasconduit 74 and cycled into the furnace of catalytic steam reformer 57 tobe burned as purge gas to recover fuel value. Pure hydrogen synthesisgas is then vented from hydrogen pressure swing adsorption purificationunit 73 via gas conduits 75 and 76 to ammonia synthesis plant 77.

Combustion exhaust gas from catalytic steam reformer 57 is ventedthrough gas conduit 78 to separation unit 79 wherein the combustionexhaust gas is separated into carbon dioxide, nitrogen, and argon richfractions according to the method of the present invention. Separationunit 79 may be a separation unit as described above in FIG. 1 or in FIG.2. Pure nitrogen synthesis gas is vented from separation unit 79 via gasconduits 80, 81, and 76 to ammonia synthesis plant 77. The pure nitrogensynthesis gas from separation unit 79 and pure hydrogen synthesis gasfrom hydrogen pressure swing adsorption purification unit 73 areemployed in ammonia synthesis plant 77 to yield ammonia according to themethod of the present invention.

Pure nitrogen gas may also be vented from separation unit 79 to nitrogenproduct reservoir 83 via gas conduits 80 and 82. Pure carbon dioxide gasis vented from separation unit 79 to carbon dioxide product reservoir 85via gas conduit 84. Pure argon gas is vented from separation unit 79 toargon product reservoir 86 via gas conduit 87. Ammonia product gas fromammonia synthesis plant 77 is vented to ammonia product reservoir 88 viagas conduit 89.

Ammonia product gas from ammonia synthesis plant 77 may also be ventedto urea synthesis plant 90 via gas conduits 91 and 92. Carbon dioxidegas from separation unit 79 may also be vented to urea synthesis plant90 via gas conduits 93 and 92. The pure ammonia product gas from ammoniasynthesis plant 77 and pure carbon dioxide gas from separation unit 79are employed in urea synthesis plant 90 to prepare urea according to themethod of the present invention. Urea product from urea synthesis plant90 is vented to urea product reservoir 94 via gas conduit 95.

In a preferred embodiment, the invention is directed at an improvedmethod for the production of ammonia which comprises the steps of:

(a) steam reforming a hydrocarbon feed gas to produce a hydrogen-richsynthesis gas;

(b) purifying the hydrogen-rich synthesis gas to remove contaminants toproduce pure hydrogen;

(c) burning a hydrocarbon fuel to supply heat for the steam reformingreaction of step (a) wherein the hydrocarbon burning produces acombustion exhaust gas containing less than about 10% oxygen by weight;

(d) treating the exhaust gas to remove particulate matter;

(e) compressing the exhaust gas to a pressure in the range from about 25psia to about 200 psia;

(f) purifying the exhaust gas to remove trace contaminants;

(g) separating the exhaust gas to produce a carbon dioxide rich fractionand a nitrogen rich fraction;

(h) liquifying the carbon dioxide rich fraction and distilling offvolatile contaminants to produce pure carbon dioxide;

(i) purifying the nitrogen rich fraction to remove contaminants;

(j) cryogenically fractionally distilling the nitrogen rich fraction toproduce pure nitrogen; and

(k) passing the pure nitrogen from step (j) and the pure hydrogen fromstep (b) into an ammonia synthesis reactor.

In another preferred embodiment, the pure carbon dioxide from step (h)is combined with the ammonia from step (k) in a urea reactor to produceurea.

As set out above, carbon dioxide and argon are preferably separated bypressure swing adsorption. In a pressure swing adsorption system (PSA),a gaseous mixture is passed at an elevated pressure through a bed of anadsorbent material which selectively adsorbs one or more of thecomponents of the gaseous mixture. Product gas, enriched in theunadsorbed gaseous component(s), is then withdrawn from the bed. Theadsorption bed may be regenerated by reducing the pressure of the bed.

The term "gaseous mixture", as used herein, refers to a gaseous mixture,such as air, primarily comprised of two or more components havingdifferent molecular size. The term "enriched gas" refers to a gascomprised of the component(s) of the gaseous mixture relativelyunadsorbed after passage of the gaseous mixture through the adsorbentbed. The enriched gas generally must meet a predetermined purity level,for example, from about 90% to about 99%, in the unadsorbedcomponent(s). The term "lean gas" refers to a gas exiting from theadsorption bed that fails to meet the predetermined purity level set forthe enriched gas. When the strongly adsorbed component is a desiredproduct, a co-current depressurization step (co-current with respect todirection of the feed gas) and a co-current purge step of the stronglyadsorbed component are added.

The selectivity of the adsorbent material in the bed for a gaseouscomponent is generally governed by the volume of the pore size and thedistribution of that pore size in the adsorbent. Gaseous molecules witha kinetic diameter less than, or equal to, the pore size of theadsorbent are adsorbed and retained in the adsorbent while gaseousmolecules with a diameter larger than the pore size of the adsorbentpass through the adsorbent. The adsorbent thus sieves the gaseousmolecules according to their molecular size, The adsorbent may alsoseparate molecules according to their different rates of diffusion inthe pores of the adsorbent.

Zeolite molecular adsorbents adsorb gaseous molecules with somedependence upon crystalline size. In general, adsorption into zeolite isfast and equilibrium is reached typically in a few seconds. The sievingaction of zeolite is generally dependent upon the difference in theequilibrium adsorption of the different components of the gaseousmixture. When air is separated by a zeolite adsorbent, nitrogen ispreferentially adsorbed over oxygen and the pressure swing adsorptionmethod may be employed to produce an oxygen enriched product. Whencarbon dioxide, nitrogen, and argon are separated by a zeoliteadsorbent, carbon dioxide is the adsorbed component and nitrogen andargon are the unadsorbed components.

The sieving action of carbon molecular sieves is generally not dependentupon differences in equilibrium adsorption but rather by differences inthe rate of adsorption of the different components of the gaseousmixture. When air is separated by carbon molecular sieves, oxygen ispreferentially adsorbed over nitrogen and the pressure swing adsorptionmethod may be employed to produce a nitrogen enriched product. Whenargon and oxygen are separated by carbon molecular sieves, argon is theunadsorbed component and oxygen is the adsorbed component.

As a gaseous mixture travels through a bed of adsorbent, the adsorbablegaseous components of the mixture enter and fill the pores of theadsorbent. After a period of time, the composition of the gas exitingthe bed of adsorbent is essentially the same as the composition enteringthe bed. This period of time is known as the break-through point. Atsome time prior to this breakthrough point, the adsorbent bed must beregenerated. Regeneration involves stopping the flow of gaseous mixturethrough the bed and purging the bed of the adsorbed components generallyby venting the bed to atmospheric or subatmospheric pressure.

A pressure swing adsorption system generally employs two adsorbent bedsoperated on cycles which are sequenced to be out of phase with oneanother by 180° so that when one bed is in the adsorption step, theother bed is in the regeneration step. The two adsorption beds may beconnected in series or in parallel. In a serial arrangement, the gasexiting the outlet end of the first bed enters the inlet end of thesecond bed. In a parallel arrangement, the gaseous mixture enters theinlet end of all beds comprising the system. Generally, a serialarrangement of beds is preferred for obtaining a high purity gas productand a parallel arrangement of beds is preferred for purifying a largequantity of a gaseous mixture in a short time cycle.

As used herein, the term "adsorption bed" refers either to a single bedor a serial arrangement of two beds. The inlet end of a single bedsystem is the inlet end of the single bed while the inlet end of the twobed system (arranged in series) is the inlet end of the first bed in thesystem. The outlet end of a single bed system is the outlet end of thesingle bed and the outlet end of the two bed system (arranged in series)is the outlet end of the second bed in the system. By using twoadsorption beds in parallel in a system and by cycling (alternating)between the adsorption beds, product gas can be obtained continuously.

Between the adsorption step and the regeneration step, the pressure inthe two adsorption beds is generally equalized by connecting the inletends of the two beds together and the outlet ends of the two bedstogether. During pressure equalization, the gas within the pores of theadsorption bed which has just completed its adsorption step (under highpressure) flows into the adsorption bed which has just completed itsregeneration step (under low pressure) because of the pressuredifferential which exists between the two beds. This pressureequalization step improves the yield of the product gas because the gaswithin the pores of the bed which has just completed its adsorption stephas already been enriched. It is also common to employ more than onepressure equalization step. When a number of pressure equalizationssteps are employed, it is common to have more than two beds in theadsorption system.

Gas separation by the pressure swing adsorption method is more fullydescribed in "Gas Separation by Adsorption Processes", Ralph T. Yang,Ed., Chapter 7, "Pressure Swing Adsorption: Principles and Processes"Buttersworth 1987, which reference is incorporate herein by reference.

Throughout this application, various publications have been referenced.The disclosures in these publications are incorporated herein byreference in order to more fully describe the state of the art.

It will be understood that the embodiments described herein are merelyexemplary and that a person skilled in the art may make many variationsand modifications without departing from the spirit and scope of theinvention. All such modifications and variations are intended to beincluded within the scope of the invention as defined in the appendedclaims.

We claim:
 1. A method for producing carbon dioxide and nitrogen fromcombustion exhaust gas containing less than about 10% oxygen by weightwhich comprises the steps of:(a) treating the exhaust gas to removeparticulate matter; (b) compressing the exhaust gas to a pressure in therange from about 25 psia to about 200 psia; (c) purifying the exhaustgas to remove trace contaminants; (d) separating the exhaust gas toproduce a carbon dioxide rich fraction and a nitrogen rich fraction; (e)liquefying the carbon dioxide rich fraction and distilling offcomponents that are more volatile than carbon dioxide; (f) purifying thenitrogen rich fraction to remove carbon dioxide; and (g) cryogenicallyfractionally distilling the nitrogen rich fraction to remove oxygen andargon therefrom.
 2. The method according to claim 1, wherein thecombustion exhaust gas contains less than about 4% oxygen, by weight. 3.The method according to claim 1, wherein the exhaust gas in step (a) istreated with a water absorption shower to remove particulate matter. 4.The method according to claim 1, wherein the exhaust gas in step (b) iscompressed to a pressure in the range from about 25 psia to about 120psia.
 5. The method according to claim 1, wherein the exhaust gas instep (c) is purified to remove sulfur oxide contaminants by treating theexhaust gas with an alkali scrubber.
 6. The method according to claim 1,wherein the exhaust gas in step (c) is purified to remove nitrogen oxidecontaminants by treating the exhaust gas with ammonia in the presence ofa selective catalyst to produce nitrogen and water.
 7. The methodaccording to claim 1, wherein the exhaust gas in step (c) is purified toremove carbon monoxide contaminants by treating the exhaust gas with anoxidation catalyst.
 8. The method according to claim 1, wherein theexhaust gas in step (c) is purified to remove trace contaminants bytreating the exhaust gas with a potassium permanganate scrubber.
 9. Themethod according to claim 1, wherein the exhaust gas in step (c) ispurified to remove water vapor by treating the exhaust gas with adesiccant.
 10. The method according to claim 1, wherein the exhaust gasin step (d) is separated by pressure swing adsorption to produce acarbon dioxide rich fraction and a nitrogen rich fraction.
 11. Themethod according to claim 1, wherein the nitrogen rich fraction in step(f) is purified to remove contaminants by passing the nitrogen fractionthrough a bed of zeolite molecular sieves.
 12. A method for producingcarbon dioxide, nitrogen, and argon from combustion exhaust gascontaining less than about 10% oxygen by weight which comprises thesteps of:(a) treating the exhaust gas to remove particulate matter; (b)compressing the exhaust gas to a pressure in the range from about 25psia to about 200 psia; (c) purifying the exhaust ga to remove tracecontaminants; (d) separating the exhaust gas to produce a carbon dioxiderich fraction and a nitrogen and argon rich fraction; (e) liquefying thecarbon dioxide rich fraction and distilling off components that are morevolatile than carbon dioxide; (f) purifying the nitrogen and argon richfraction to remove carbon dioxide therefrom; (g) cryogenicallyfractionally distilling the nitrogen and argon rich fraction to producepure nitrogen and an argon rich fraction; and (h) purifying the argonrich fraction to produce pure argon.
 13. The method according to claim12, wherein the combustion exhaust gas contains less than about 4%oxygen, by weight.
 14. The method according to claim 12, wherein theexhaust gas in step (b) is compressed to a pressure in the range fromabout 25 psia to about 120 psia.
 15. The method according to claim 12,wherein the exhaust gas in step (d) is separated by pressure swingadsorption to produce a carbon dioxide rich fraction and a nitrogen richfraction.
 16. The method according to claim 12, wherein the nitrogenrich fraction in step (f) is purified to remove contaminants by passingthe nitrogen fraction through a bed of zeolite molecular sieves.
 17. Themethod according to claim 12, wherein the argon rich fraction in step(h) is purified by pressure swing adsorption.
 18. The method accordingto claim 12, wherein the argon rich fraction in step (h) is purified bycryogenic distillation.